This invention relates to a sewing machine, and more particularly to a bobbin thread detection apparatus for sewing machines.
Lockstitch sewing machines conventionally include a small amount of bobbin thread wound on a bobbin. Since the thread is used up in a relatively short period of time, an inspection is frequently required to determine how much thread remains on the bobbin. Such inspection has been generally made by tilting a head of the sewing machine. However, this extra labor inevitably lowers the efficiency of the sewing operation. For this reason, various approaches have been made to automatically inspect how much bobbin thread remains wound on a bobbin when the residual or leftover amount of bobbin thread is less than a predetermined minimal amount.
Apparatuses which measure the amount of residual bobbin thread are broadly classified into those which are adapted for mechanical inspection and those which are adapted for optical inspection. The mechanical inspection apparatuses are used, for example, when the user is required to inspect the residual or leftover bobbin thread while it is wound on the bobbin such as when detection elements are adjusted according to variations depending upon the condition of the workpiece and the sewing operation. This operation could not be performed unless the head of the sewing machine is tilted, which requires the use of much operator labor.
Lately, there has been a tendency that bobbin thread inspections be made using an optical sensor. For example, one conventional optical inspection apparatus for detecting the residual or leftover bobbin thread, as shown in FIGS. 10A and 10B, is provided with an optical sensor C. More specifically, the apparatus of this class comprises a bobbin B which includes a flange Ba having a plurality of reflective elements B1 radially and equidistantly formed thereon. The reflective elements B1 of the flange Ba are adapted to irradiate light from an illuminator C1 to the flange Ba. A light receptive element C2 converts the light which is continuously reflected from the reflective elements B1 into a pulse signal.
When the main shaft rotates one-half turn, the number of pulse signals generated is detected, thereby determining the rotation speed of the bobbin. This measured bobbin rotation speed is then compared with the rotational speed of a bobbin which contains the preset minimum amount of thread. If the measured rotational speed is more than the preset speed, the operator is notified that the residual or leftover bobbin thread remaining is low, so that a new bobbin may be applied.
In other words, to cope with the aforementioned situation where the rotation speed of the bobbin is increased as the residual or leftover thread is reduced, the rotation speed of the bobbin is measured to determine whether or not the amount of residual or leftover thread is less than the present minimum amount.
However, the aforementioned apparatus as those which are adapted so that the amount of the residual or leftover thread is detected by the rotation speed of the bobbin B, involve a disadvantage in that the rotation speed of the bobbin is subjected to the coefficient of friction between the bobbin B and a bobbin case (not shown), thus necessitating resetting its rotation speed when another bobbin is exchanged therewith.
Another disadvantage is that the rotation speed of the bobbin varies considerably even where the main shaft rotates one revolution, resulting in unreliable speed data due to instability, thereby resulting in malfunction. For instance, the bobbin B is initially rotated at a high speed immediately after the bobbin thread is pulled up by a needle thread loop, but is thereafter decelerated since its rotation entirely relies upon inertia, thus causing a great change in its speed. Moreover, sewing speed is not always constant during one sewing cycle and the rotation speed of the bobbin also varies considerably by a change in the rotation speed of the main shaft.
For these reasons it is very difficult to precisely detect the amount of the residual or leftover thread, thereby resulting in many malfunctions.
It is therefore an object of the present invention to eliminate the aforementioned disadvantages of the conventional apparatus for optically detecting the amount of the residual or leftover bobbin thread.
Another object of the invention is to provide an apparatus for detecting the amount of the residual or leftover bobbin thread, which is capable of reliably detecting the amount of the bobbin thread when the remaining thread reaches a predetermined minimal amount.